I was having problems with GTK applications (mostly Emacs and Ardour) producing some glitches, like parts of the screen flickering and not being displayed properly.

After some Googling, I found a suggestion to change AccelMethod from "sna" to "uxa" in /etc/X11/xorg.conf.d/20-intel.conf. It worked, it even fixed a problem that, despite my Xorg and xrandr, configurations, Plasma would switch the primary and secondary displays.

BUT: now when the monitors turn off (the computer does NOT suspend, only the screens turn off), I cannot turn them on again until I switch to and a back from a virtual terminal (Ctrl+Alt-F1 then Ctrl+Alt-F7). Then, all is back to normal.